The aim of this study was to investigate subclinical abnormalities in renal function suggestive of incomplete distal renal tubular acidosis (DRTA) in patients with rheumatoid arthritis (RA), using the gradient between PCO2 in urine and blood (U-B PCO2 gradient), which is a simple and sensitive test. We prospectively selected 45 patients in three groups (G). G 1 (n = 15p), with RA, mean age 44 years and mean disease duration 6.5 years. G2 (n = 10 p), with RA and Sjögren's syndrome (SS), mean age 47 years and mean disease duration 6.6 years. G 3 (n = 20) healthy volunteers, no history neither renal nor rheumatic diseases, mean age 37 years. Patients in G1 and G2 had no history of concurrent disease affecting renal parenchyma, their acid-base status and renal function were normal (Creatinine clearance above 70 ml/min/1.73m2). All patients received NSAIDs but none gold salts and/or D-Penicillamine. The ability to acidify the urine was evaluated in all cases by U-B PCO2 gradient, after a 500 ml NaHCO3 continuous infusion 1 molar solution through a peripheral vein. U-B PCO2 lower than 30 was considered pathological and diagnostic for DRTA. The urinary specimen for pH and PCO2 were kept under mineral oil and processed within 5 minutes of excretion. The blood samples for PCO2 were obtained from a peripheral vein and measured after obtaining a urinary pH 7.8 or above, pH and PCO2 were measured with a BMS 3 MK2 Radiometer, Copenhagen Denmark electrode and analizer. The U-BPCO2 results were (mean 2 sd): G1 = 47 +/- 26; G2 = 49.8 +/- 8.4; G3 = 52.5 +/- 12.2. There were no statistical differences among the three groups (F = 1.228727). In the G1, a single patient presented U-B PCO2 lower than 30 (U-B PCO2 = 5), having a long active disease at the evaluation time. In G2 and G3 no gradient alterations were recorded. We conclude, in spite of the fact that U-B PCO2 gradient is a very useful and sensitive tool for detecting dRTA, that there was no correlation between incomplete type dRTA and RA or between incomplete dRTA and RA associated to SS. In addition, no association was found between NSAID intake and dRTA.